1. Field of the Invention
This present invention relates to an improved method of producing an injection molded article having a hollow blown rim being an integrated part of the article.
2. Discussion of the Prior Art
Injection molded container, crates, pallets and the like have most often one or more upper or outer rims whereby the rim provides strength and stability, as well being a stacking or gripping means. A commonly used concept for a rim consists of at least one U-shaped and downward open profile being circumambient or located on at least two opposite sides of a container, etc. In order to obtain satisfactory strength, rims of this type require reinforcing and other structural means, such as ribs, girders, webs, edgings, rippings and the like, which means are placed inside the rim or connecting the rim with a side wall of a container, a loading area of a pallet, etc. The manufacture of injection molded articles has such a rim concept and, furthermore, incurs a complex design of the mold cavities which are included in the mold, an excessive material usage and an increased cycle time. This results in an unnecessarily high product weight and in unnecessarily high costs. Open rims also cause and affect the accumulation of various contaminants, which is a major problem, especially in food processing and food transporting.
This problem can, however, be remedied by injecting, for example, a gas through separate inlets or together with the molten polymeric material. It is possible to thereby decrease the weight of the part and to make the ribs and profiles thicker, as seen from the outside. It is also known according to the state of the art that a closed hollow profile is stronger than an open profile, as seen, for example, in a traditional rib with the same amount of material used. Thus, the number of ribs and profiles can be reduced such that the product will become easier to clean.
One disadvantage of gas channels achieved through injection of a gas together with polymeric material is that the thickness of the material in the part is very difficult to guide. This is due to the fact that the polymeric material is very soft in a molten or semi-molten state. This might even cause a so-called blowout, wherein the gas cavity within the polymeric material ruptures the bubble of molten polymeric material. This occurs because the gas pressure is higher than the molding pressure. Gas injection pressures of up to 10 MPa are not unusual. Since the gas is normally injected through very narrow nozzles, the injection of the gas easily causes ruptures and uneven material thickness caused by turbulence in the hollow profiles. When a rupture occurs, the article will have to be rejected, and uneven material thickness will cause unreliability in the quality of the product, since wear could open thinner wall parts of the closed hollow profile. It is also known according to the state of the prior art that the undesirable thinner wall parts of the profile act as an indication of fracture, which randomly affects the mechanical strength of the product.
Hollow profiles can be produced using a number of well-known gas injection procedures which have been disclosed in detail. Some of the most important and commonly used procedures and companies or origin, include: The Battenfeld AIRMOLD, the Klockner Ferromatik AIRPRESS, the Pearlesss Cinpress "Controlled Internal Pressure Moulding" (CINPRES), Encore Systems' "Engineered Nitrogen Coresystem" (ENCORE), Detroit Plastic Molding's "Gas-assisted Injection Molding" (GAIN), Engel's GASMELT, Dynamit-Nobel's "Gasinnendruckvergahren" (GID), Demag's "Gasinnendruckproze.beta." (GIP)and Schade's "Gasinjektiontechnik" (GIT).
A common feature of gas injection procedures encompass a mold cavity of a mold filled with a molten polymeric material followed by injection of a pressurized gas. Parts of the molten polymeric material are thereby displaced by the gas due to the discharge of excess of material or the expansion of the mold cavity, which results in a hollow, substantially closed molded article or a part of such an article. The pressurized gas is normally injected into the core of the molten polymeric material.
Gas injection procedures are used in conjunction with the injection molding of containers, crates, pallets and the like, as well as in the manufacture of e.g., pallet skids, various gripping and reinforcing means and partial rims, etc. International Publication No. WO 90/00466 discloses a method which allows articles of relatively large dimensions to be produced using a gas injection procedure. The method comprises a mold provided with a spill cavity. A charge of pressurized gas is injected into the mold cavity to displace a portion of the molten material. Displaced material flows through a passage into a connected spill cavity or reservoir alternatively serving as an appendage of the article, a separate article or a cavity to receive spilled material for regrinding. Such cavities necessitate a certain amount of subsequent treatment, such as removal of separate articles, spill material and strings, bar and the like connecting the finished, molded article with the material or article in a spill cavity.